Viscosity vs Density
Viscosity and density are two properties of liquids and gases (or referred as fluids). They are very useful physical quantities when it comes to describing statics and dynamics of these substances. Viscosity and density alone can describe more than half the properties of a fluid.
Viscosity
Viscosity is defined as a measure of the resistance of a fluid which is being deformed by either shear stress or tensile stress. In more common words, viscosity is the “internal friction” of a fluid. It’s also referred as the thickness of a fluid. Viscosity is simply the friction between two layers of a fluid when the two layers move relative to each other. Sir Isaac Newton was a pioneer in fluid mechanics. He postulated that, for a Newtonian fluid, the shear stress between layers is proportional to the velocity gradient in the direction perpendicular to the layers. The proportional constant (proportionality factor) used here is the viscosity of the fluid. The viscosity is usually denoted by the Greek letter “µ”. Viscosity of a fluid can be measured using Viscometers and Rheometers. The units of viscosity are Pascal-seconds (or Nm-2s). The cgs system uses the unit “poise”, named after Jean Louis Marie Poiseuille, to measure viscosity. Viscosity of a fluid can also be measured by several experiments. The viscosity of a fluid depends on the temperature. The viscosity decreases as the temperature is increased.
Viscosity equations and models are very complex for non-Newtonian fluids.
Density
Density is defined as mass per unit volume. Density plays a vital role in fluid mechanics. Events such as up thrust are dependent on the density. Density is what we normally refer to as “weight” of a fluid. Density is a concept with which we are really familiar. It can be obtained from the simple equation density=mass/volume. Its units are Kgm-3.
What is the difference between Viscosity and Density?
While most of the people think that viscosity and density are both the same thing expressed in different forms, they are two truly different concepts. The density is a measurement of the molecular weight of the composition. In simpler words, density = number of molecules x molecular weight/volume occupied, while the viscosity is a measurement of the inter-molecular forces and molecule shapes. Viscosity tells you the “friction” between two layers of the given fluid, while density varies slightly with temperature, viscosity changes rapidly. Both density and viscosity decreases with temperature, but viscosity mostly has an exponential relationship with temperature. Density holds a linear relationship. This temperature viscosity relationship is the base of the auto lubricant technology.
Viscosity and density are two different physical phenomena depending on totally different aspects. The common misconception of “heavier fluids are more viscos” is to be omitted.